In the field it is common to store multiple riser sections from which the subsea riser string is composed in a riser storage of the vessel.
Commonly a riser section comprises a main riser pipe and in many known embodiments additionally one or more auxiliary pipes, also often identified as service, satellite, or peripheral pipes. The auxiliary pipes extend on the outside of and along the main riser pipe. The auxiliary pipes e.g. include a choke line, a kill line, one or more hydraulic lines, e.g. used as fluid lines to a BOP or other subsea equipment, booster lines, injection lines (e.g. for glycol), etc. Each riser section comprises a connector fitting arrangement at each end thereof. For example the connector fitting arrangement includes a flange having bolt holes, with riser sections being joined by interconnecting flanges by means of bolts. An auxiliary pipe may have an individual connector end fitting, e.g. a bayonet fitting, or be designed to fit sealingly into the auxiliary pipe of an adjoining riser section without direct axial securing of the auxiliary pipes. In practical embodiments a riser section is often provided with one or more buoyancy members and/or thermal insulation members, e.g. of plastic foam material, but so-called bare joints are also employed.
Riser sections come in different lengths. Commonly riser sections have lengths between 50 ft. (15.24 meters) and 90 ft. (27.43 meters). A most common length is 75 ft. (22.86 meters).
Riser sections are commonly heavy; far heavier than other tubulars used in the offshore drilling industry. For example a single 75 ft. subsea riser section may weigh between 20 and 25 tonnes, which is incomparable to the weight of an equally long drill pipe. Therefore riser handling is subject to different considerations than drill pipe handling, mainly in view of their size and weight.
For example WO2009/102196 discloses a mono-hull vessel having a hull and a riser storage hold within the hull. In the riser storage hold riser sections are stacked in horizontal orientation. A crane is provided to raise and lower the riser sections out of and into the storage hold and to place each individual riser section onto a riser catwalk machine or to pick up a riser section from the catwalk machine. The leading end of the riser section is in practice connected to a riser string lifting tool which connects the riser section to a riser string capacity hoisting device of the vessel. By raising the lifting tool and operation of the catwalk machine the riser section is brought into a vertical orientation, or upended, in line with a firing line along which the riser string is suspended into the sea. The already launched portion of the riser string is then temporarily held by a riser string hanger, often referred to as a riser spider device, of the vessel. The new riser section is then held in alignment above the launched riser string and the connector fitting arrangements are interconnected to join the new riser section to the riser string. Then the riser string is released by the riser spider device and lowered over the length of the newly attached section. The riser string is then suspended again from the riser spider device and the process of adding and joining a new riser section is repeated.
In WO2014/168471 it is discussed that this process to assemble a riser string is time-consuming and in view thereof the use of significantly longer riser sections is proposed. For example this prior art document proposes the use of individual riser sections, or pre-assembled riser sections called stands, that have a length of between 100 ft. (30.48 m) and 180 ft. (54.86 m), e.g. of 120 ft. (36.57 m). A prominent example is a length of 150 ft. (45.72 m).